Regulation of osteoclastogenesis and arthritic bone resorption by RBP-J. Osteoclasts, derived from monocyte/macrophage precursors, are the exclusive bone resorptive cells that play an important role not only in physiological bone development and remodeling, but also function actively as a key pathogenic cell leading to musculoskeletal tissue damage and accelerating pathogenesis of diseases characterized by inflammatory osteolysis, including rheumatoid arthritis (RA), psoriatic arthritis, periodontitis an peri-prosthetic loosening. This application will focus on the mechanisms that restrain excessive osteoclastogenesis and arthritic bone resorption in inflammatory settings. Osteoclastogenesis is delicately controlled by positive and negative regulatory mechanisms. In contrast to the extensive study of the positive regulation of osteoclastogenesis, the feedback inhibitory mechanisms that negatively regulate the magnitude of osteoclast formation and function, especially in pathological conditions, are less appreciated. Augmentation of these mechanisms represents potential approaches to inhibiting excessive osteoclastogensis and bone resorption. The candidate's long term goals are to identify and understand the homeostatic and feedback inhibitory mechanisms during osteoclastogenesis, and to utilize this knowledge in the development of new therapeutic approaches to diseases associated with inflammatory osteolysis. The candidate is particularly interested in TNF-1 mediated osteoclastogenesis and bone destruction because TNF-1 is a key pathogenic factor driving inflammatory bone resorption. Therefore, the candidate has initiated studies to identify the mechanisms that restrain TNF-1-induced osteoclast differentiation and bone resorption. The candidate has recently identified the transcription factor RBP-J that is activated by TNF-1 stimulation and dramatically suppresses TNF-1-induced osteoclastogenesis and bone resorption in vitro and in vivo but has minimal effects on physiological bone remodeling. This indicates that RBP-J is a key negative regulator of inflammatory/pathologic bone resorption and thus an attractive therapeutic target for arthritis associated with bone destruction, such as RA. The goals of the proposed research are: 1) to reveal molecular mechanisms by which RBP-J negatively regulates TNF-1 induced osteoclastogenesis, including the transcriptional regulation of NFATc1 and mediation of the transcriptional repressor network by RBP-J, and 2) to identify the role of RBP-J in inflammatory arthritic bone resorption, including the significance of therapeutic targeting of RBP-J to suppress inflammatory bone resorption in arthritis animal models. The candidate anticipates that the proposed studies will yield insight into mechanisms that restrain pathologic osteoclastogenesis and inflammatory osteolysis, and will be useful in developing new therapeutic approaches to suppressing bone resorption in inflammatory settings such as occurs in RA.